Kris, I can't imagine anyone at Cingular would care if you're using a DASH. I've run iMates and other GSM stuff from the UK on T-Mo, and before that on ATT and was always told, essentially, "it's your contract, you can run whatever you want if it works." Only time it mattered was when I asked them for support setting up WAP when they changed everything 2-3 years back...even then, T-Mo escalated it to tier 3 tech support, who were totally cool.
Re the push icon....I'm not running Exchange (hence mine's disabled), but if you set up auto-notify in T-Mo's WAP email -- and disable text message notifications -- it pushes mail through anyway. I've got 2 POP and 2 IMAP boxes and they're all working.